OVA file not found fails ungracefully

[michaelrice]

When a file is not found due to bad path etc a ValueError is raised. This should be caught by the execute method and handled in a clean way.

Traceback (most recent call last):
  File "/Users/errr/venvs/simdeplower/bin/simdeplow", line 9, in <module>
    load_entry_point('simdeplower==0.1', 'console_scripts', 'simdeplow')()
  File "/Users/errr/venvs/simdeplower/lib/python2.7/site-packages/simdeplower/executable.py", line 141, in execute
    name=provided_args.name)
  File "/Users/errr/venvs/simdeplower/lib/python2.7/site-packages/simdeplower/ovfhelper.py", line 63, in extract_ovf_to_file_system
    raise ValueError("Invalid source location supplied. File not found.")
ValueError: Invalid source location supplied. File not found.

This should just say something like:

OVA file not found. Check path